Very feature complete game. Quite fun. Nice graphics.
Can't see how it is trying to exploit the limitations of "Only one".
Given the low difficulty of just pressing a key,
perhaps you can stylize some characters and let the player spend time guessing it?
or perhaps let the player be the enemies and press the button to send a fast ship crashing into a shielded computer player (sitting in the center)?
or make the player use arrow keys to stay away from the (even denser) enemies, and use only one hand to type the character?
Still, really enjoyed the lighting and atmosphere you have created.